Polyimides are generally excellent in heat resistance, antistatic property and mechanical properties, among other characteristics, and are used in various fields. Particularly, in the field of laser printers and electrophotographic copying machines, for instance, the use of them as parts or members (rolls or belts) for heating for fixation of toner images on toner image acceptors (paper, carton, OHP (overhead projector) sheets, etc.) is under investigation. In using them as such heating and fixing members, they are required to be high in thermal conductivity as well. Organic polymers are essentially low in thermal conductivity and rather heat-insulating. The same is true of polyimides.
Therefore, various measures have been taken to increase the thermal conductivity. A simple approach toward this goal is to reduce the thickness. This indeed increases the thermal conductivity but can meet sophisticated requirements only to a limited extent. In addition, it affects mechanical characteristics (in particular flex resistance, scratch resistance, etc.) adversely. Therefore, it can never be a satisfactory means of solution.
As an attempt to essentially improve the thermal conductivity, it has been proposed to admix a known thermal conductivity providing agent with polyimides, as can be seen, for example, in Japanese Kokai Tokkyo Koho S62-3980, H07-110632 and H07-186162.
It is common to the technologies disclosed in the above-cited references to employ a heat-conductive cylindrical polyimide film containing a thermal conductivity providing agent uniformly dispersed therein as an inner layer and laminate thereon an outer layer of a resin such as a non-heat-conductive polyimide or a fluororesin, for the purpose of heating and fixation in copying machines.
Meanwhile, in mixing various additives in general, inclusive of the thermal conductivity providing agent mentioned above, with various resins, inclusive of polyimides, it is an essential goal to attain as uniform mixing and dispersion as possible. For promoting the expression of required performance characteristics, it is also necessary to increase the amounts of the additives. The addition of the additives indeed markedly improves the performance characteristics but antinomically deteriorates the mechanical characteristics inherent in the resins. This applies to the technologies of the above-cited references as well. A means for preventing said characteristics deterioration is to increase the film thickness. This, however, tends to make the film more rigid and deficient in flexibility, resulting in loss of the film characteristics.